Hypophosphatemia

Evaluation: 

• Serum phosphorus levels should be interpreted with concomitant evaluations of PTH, serum total and ionized calcium, urine phosphate and calcium and Vitamin D
• Medical history, physical examination and lab tests to determine etiology of hypophosphatemia (renal wasting, decreased intestinal absorption etc)
• Concomitant serum levels of Magnesium and Potassium should be checked as well and replaced per protocol 

Causes:

• Hyperparathyroidism 
• Vitamin D deficiency
• Malabsorption/emesis 
• Increased urinary secretion
• Refeeding after malnutrition
• Hyperalimentation
• Recovery of DKA, effects of insulin/glucagon/androgens
• Hypothermia (recovering from hypothermia) 
• Steatorrhea 
• Medication (e.g. phosphate binders, salicylate poisoning, steroids, diuretics) 

• Decreased intake
• Alcohol abuse, withdrawal
• Renal tubular defects (aldosteronism, SIADH)
• Sepsis
• Gout 
• Burns 
• Respiratory alkalosis
• Hypomagnesemia 

Signs: Generally seen with serum phosphate levels below 1 mg/dl 

• Numbness
• Weakness 
• AMS / confusion
• stupor
• Seizures 
• coma
• Muscle pain 
• Rhabdomyolysis 
• Resp. failure 
• CHF
• Paresthesias
• Dysarthria 
• Hemolysis 
• Platelet dysfunction
• Metabolic acidosis 

Therapy:

• Treat underlying cause, the phos levels will normalize automatically ( DKA, diarrhea, chronic antacid therapy, or vitamin D deficiency) 
• Replete if serum phos levels are less than 2.0 mg/dl (0.64 mmol/L)
• Keep phos levels over 1 mg/dl, above that, oral replacement is preferable to avoid hyperphosphatemia ( which can cause subsequent hypocalcemia, ectopic calcifications, renal failure, or hypotension)
• In Asymptomatic patients with serum levels below 2.0 mg/dl replete with oral phosphate

- Serum level of 1.5 mg/dl (0.48 mmol/L) → give 1 mmol/kg elemental phosphorus (minimum of 40 and maximum of 80 mmol can be given in 4 doses over 24h) 

- Serum level less than 1.5 mg/dl → give 1.3 to 1.4 mmol/kg of elemental phosphorus (up to a maximum of 100 mmol given in 4 doses over 24h) 

• In Symptomatic patients treatment varies with severity of the hypophosphatemia 

  - Serum level of 1.0 to 1.9 mg/dl (0.32 to 0.63 mmol/L) → treat with oral phosphate 

- Serum level less than 1.0 mg/dl → treat with IV phosphate and switch to oral 

- Stop replacement when serum levels is equal to or greater than 2.0 mg/dl

• IV preparations: IV phosphate is potentially dangerous. If IV phos is necessary, use dose depending on severity of hypophosphatemia and the weight of the patient. 

- If the serum phosphate concentration is greater than or equal to 1.25 (0.40 mmol/L), → give 0.08 to 0.24 mmol/kg over six hours (up to a maximum total dose of 30 mmol).

- If the serum phosphate concentration is less than 1.25 mg/dL (0.40 mmol/L) → give 0.25 to 0.50 mmol/kg over 8 to 12 hours (up to a maximum total dose of 80 mmol).

→ measure serum phosphate levels Q6H and switch the oral replacement once serum levels have reached 1.5 mg/dl (0.48 mmol/L)

• Use either sodium phosphate or potassium phosphate depending on whether patient also needs potassium 
• Follow Potassium and Magnesium as well and replete per protocol 

https://www.uptodate.com/contents/hypophosphatemia-evaluation-and-treatment?search=hypophosphatemia&source=search_result&selectedTitle=1~150&usage_type=default&display_rank=1#H727383

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4324571/#:~:text=When%20evaluating%20a%20patient%20with,not%20reliable%20indicators%20of%20total